1. Field of the Invention
The present invention relates to a printed wiring board composed of a wiring line formation layer and wiring lines having a different thickness formed on a surface of the wiring line formation layer.
2. Description of the Related Art
In general a printed wiring board is composed a wiring line formation layer and a plurality of wiring lines. These wiring lines have a different thickness and are formed on a surface of the wiring line formation layer.
The wiring lines having a different thickness make it possible to provide a function to flow a large current supplied from a power element flows through a wiring line having a large thickness. Through the wiring line having a large thickness discharges thermal energy. On the other hand, signals transferred from various types of control elements are transferred through a fine wiring line having a small thickness.
For example, a conventional document, Japanese patent laid open publication No. JP 2001-156408 discloses such a printed wiring board in which a first wiring line and a second wiring line having a same thickness are formed on a surface of a wiring line formation layer by performing patterning process to make wiring lines. Then, etching or filling is performed for one which is selected from the first wiring line and the second wiring line in order to increase the thickness of the selected wiring line.
However, in the conventional technique disclosed by the above conventional document, Japanese patent laid open publication No. JP 2001-156408, etching or filling is performed twice for the selected wiring line. That is, the conventional technique requires the step of performing the patterning process at least twice. This would often cause variation in dimension of the wiring line in the patterning formation process.
The inventors according to the present invention study the method to form wiring lines on a surface of a wiring line formation layer in a printed wiring board by using a patterning formation step one time. That is, a first wiring line and a second wiring line having a different thickness are formed by using the patterning formation step only one time.
FIG. 18 shows a schematic cross section of the printed wiring board made by the above method.
As shown in FIG. 18, the printed wiring board is a laminated substrate which is composed of a wiring line formation layer 1810 as an inner core. A first wiring line 1830 having a large thickness is formed on one surface (upper surface or front surface) of the printed wiring board. An insulation layer 1820 is formed on the surface, with which the entire surface of the wiring line formation layer 1810 including the first wiring line 1830 is covered. A second wiring line 1831 having a thickness which is smaller than the thickness of the first wiring line 1830. In the structure of the printed wiring board shown in FIG. 18, the first wiring line 1830 having a large thickness is composed of multi layers.
In the structure of the printed wiring board shown in FIG. 18, the insulation layer 1820 is formed on the first wiring line 1830 having a large thickness, and the second wiring line 1831 having a small thickness is formed on the insulation layer 1820. As shown in FIG. 18, this structure increases the entire thickness of the printed wiring board by the thickness of the insulation layer 1820 because the insulation layer 1820 is formed between the first wiring line 1830 and the second wiring line 1831.